Grafted crosslinked cellulose

ABSTRACT

Grafted, crosslinked cellulosic materials include cellulose fibers and polymer chains composed of at least one monoethylenically unsaturated acid group-containing monomer (such as acrylic acid) grafted thereto, in which one or more of said cellulose fibers and said polymer chains are crosslinked (such as by intra-fiber chain-to-chain crosslinks). Some of such materials are characterized by a wet bulk of about 10.0-17.0 cm 3 /g, an IPRP value of about 1000 to 7700 cm 2 /MPa·sec, and/or a MAP value of about 7.0 to 38 cm H 2 O. Methods for producing such materials may include grafting polymer chains from a cellulosic substrate, followed by treating the grafted material with a crosslinking agent adapted to effect crosslinking of one or more of the cellulosic substrate or the polymer chains. Example crosslinking mechanisms include esterfication reactions, ionic reactions, and radical reactions, and example crosslinking agents include pentaerythritol, homopolymers of the graft species monomer, and hyperbranched polymers.

TECHNICAL FIELD

This disclosure relates to cellulose fibers, and in particular tografted, crosslinked cellulose.

BACKGROUND

Cellulosic fibers find utility in many applications, includingabsorbents. Indeed, cellulosic fibers are a basic component of manyabsorbent products such as diapers. The fibers form a liquid absorbentstructure, a key element in an absorbent product.

Cellulosic fluff pulp, a form of cellulosic fibers, has been used forabsorbent applications because the fluff pulp form provides a high voidvolume, or high bulk, liquid absorbent fiber structure. However, thisstructure tends to collapse upon wetting, which reduces the volume ofliquid that can be retained in the wetted structure. Further, suchcollapse may inhibit transfer of liquid into unwetted portions of thecellulose fiber structure, leading to local saturation.

Whereas the ability of an absorbent product containing cellulosic fibersto initially acquire and distribute liquid (such as from an initialliquid insult) relates to the product's dry bulk and capillarystructure, the ability of a wetted structure to acquire additionalliquid (such as from subsequent and/or extended liquid insults) relatesto the structure's wet bulk. Due to diminished acquisition and capacityproperties related to loss of fiber bulk associated with liquidabsorption, the potential capacity of a dry high bulk fiber structuresuch as cellulosic fluff pulp may not be fully realized, with the liquidholding capacity instead determined by the structure's wet bulk.

Intra-fiber crosslinked cellulose fibers and structures formed therefromgenerally have enhanced wet bulk as compared to non-crosslinked fibers.The enhanced bulk is a consequence of the stiffness, twist, and curlimparted to fibers as a result of crosslinking. Accordingly, crosslinkedfibers are incorporated into absorbent products to enhance their wetbulk and liquid acquisition rate.

In addition to wet bulk and liquid acquisition rate, a material'ssuitability for use in absorbent products may be characterized in termsof other performance properties, such as liquid permeability. As notedabove, performance properties tend to result from different fibercharacteristics such as fiber length, fiber stiffness, and so forth.However, relationships between some performance properties indicate theexistence of trade-off trends for many cellulose fiber (and other)materials. For example, liquid permeability tends to decrease ascapillary pressure, expressed in terms of medium absorption pressure,increases. As explained in greater detail below, this particularrelationship manifests in a manner that can be mathematicallyapproximated as a power curve function of the two properties, which ischaracteristic for many if not all materials used in absorbentapplications, including cellulose fiber materials, synthetic fibermaterials, blends, and so forth. Of these materials, the “trade-off”curve for cellulose fiber products is the highest, but successfulefforts to raise this curve higher—that is, to produce materials thatexhibit better liquid permeability value at a given capillary pressurevalue (and vice versa) than as predicted by the power curve functiondescribed by cellulose fibers—have not yet been observed.

There are a number of methods for preparing crosslinked cellulosefibers; several are summarized in U.S. Pat. No. 5,998,511 to Westland,et al. Much effort has been spent improving crosslinking processes, suchas to lower production and/or material costs, to modify absorbent and/orother fiber properties of the products, and so forth. In one example,polycarboxylic acids have been used to crosslink cellulosic fibers (suchas in U.S. Pat. Nos. 5,137,537, 5,183,707, and 5,190,563, all to Herron,et al., and so forth), to produce absorbent structures containingcellulosic fibers crosslinked with a C2-C9 polycarboxylic acid. Despiteadvantages that polycarboxylic acid crosslinking agents provide,cellulosic fibers crosslinked with low molecular weight (monomeric)polycarboxylic acids, such as citric acid, have been found to undergoreversion to a non-crosslinked condition and thus have a usefulshelf-life that is relatively short. Polymeric polycarboxylic acidcrosslinked fibers, however, such as disclosed in U.S. Pat. Nos.5,998,511, 6,184,271, and 6,620,865, all to Westland, et al., amongstothers, resist such aging or reversion, due in part to the participationof the polymeric polycarboxylic acid molecule in the crosslinkingreaction with an increased number of reactive carboxyl groups than isthe case with monomeric polycarboxylic acids such as citric acid. Inanother example, U.S. Pat. No. 8,722,797 to Stoyanov, et al., disclosesthe use of a comparatively low molecular weight polyacrylic acid havingphosphorous (in the form of a phosphinate) incorporated into the polymerchain as a crosslinking agent to achieve crosslinked cellulose fibershaving improved brightness and whiteness (as well as other properties)as compared to those prepared with higher molecular weight phosphinatedagents or polyacrylic acid agents without phosphinates.

Thus, there is a continuing need to produce crosslinked cellulose fibersand compositions and materials including such fibers suitable for use inabsorbent and other applications.

SUMMARY

Various embodiments of crosslinked cellulosic materials composed of acellulosic substrate, such as cellulose fibers, having grafted polymerchains, with such cellulose fibers and/or grafted polymer chains beingcrosslinked, and various methods of producing such compositions, aredisclosed herein. The fibrous materials produced according to thepresent disclosure are also referred to herein as “grafted crosslinkedcellulose” and may be described as “compositions” as well as“materials.” In embodiments of the grafted crosslinked cellulose of thepresent disclosure, the polymer chains are composed of monoethylenicallyunsaturated acid group-containing monomers (non-limiting examplesinclude acrylic acid, maleic acid, methacrylic acid, etc., andcombinations thereof), which may be crosslinked in a variety of manners(non-limiting examples include ester intra-fiber crosslinks viacrosslinking agents such as homopolymers, hyperbranched polymers,pentaerythritol, and so forth).

In some embodiments, acrylic acid is used as the monoethylenicallyunsaturated acid group-containing monomer. Some embodiments arecharacterized by a graft yield of 5-35 weight %, and more particularly10-20 weight %. Some embodiments are characterized by a wet bulk atleast 6% greater, and up to at least 40% greater, than untreatedcellulose (referring to the cellulose substrate in an untreated, i.e.,non-grafted and non-crosslinked, state). Some embodiments arecharacterized by a wet bulk of about 10.0-17.0 cm³/g, and moreparticularly of about 15.0-17.0 cm³/g. Some embodiments include mainlyintra-fiber chain-to-chain crosslinks composed of a crosslinking agentsuch as pentaerythritol or a hyperbranched polymer. Some embodimentsinclude intra-fiber chain-to-cellulose crosslinks.

In some of the aforementioned embodiments, the material is characterizedby an IPRP value of about 1000 to 7700 cm²/MPa·sec and a mediumabsorption pressure (MAP) of about 7.0 to 20 cm H₂O. Some embodimentsare further characterized by power curve function wherein for a givenIPRP value y (in cm²/MPa·sec) from 1000 to 7700, the MAP value of thematerial (in cm H₂O) is within +/−30% of the value of x in the formulay=mx^(Z); wherein m is from 600 to 1200, and wherein z is from −0.590 to−0.515. Some embodiments are more particularly characterized in that zis from −0.560 to −0.520 and/or m is from 800 to 1100. In someembodiments, at a given IPRP value (in cm²/MPa·sec) from 800 to 5400,the material has a MAP value that is equal to or higher (e.g., 0-20%higher) than the corresponding MAP value possessed by non-grafted,crosslinked cellulose fiber, and/or at a given MAP value (in cm H₂O)from 7.0 to 20, the material has an IPRP value that is equal to orhigher (e.g., 0-15% higher) than the corresponding IPRP value possessedby non-grafted, crosslinked cellulose fiber. Some embodiments have anIPRP value of 5400 cm²/MPa·sec or above.

Example methods of producing grafted crosslinked cellulose in accordancewith the present disclosure include grafting polymer chains of at leastone monoethylenically unsaturated acid group-containing monomer from acellulosic substrate to produce a grafted cellulosic material, followedby crosslinking the grafted cellulosic material by treating the materialwith a crosslinking agent adapted to effect crosslinking of one or moreof the cellulosic substrate or the polymer chains. In some methods, thegrafting is performed in situ and may include reacting the monomer withthe cellulosic substrate in the presence of a grafting initiator such ascerium(IV) sulfate. In some methods, acrylic acid is used as themonomer. Some methods include varying the amounts of the initiatorand/or the ratio of cellulose to monomer to achieve a desired graftlevel.

Such methods may include any of a variety of crosslinking procedures.Some methods include establishing intra-fiber crosslinks via anesterification reaction via one or more crosslinking agents such aspentaerythritol, a polymeric crosslinking agent (for example, ahomopolymer formed of the at least one monoethylenically unsaturatedacid group-containing monomer), a hyperbranched polymer, and so forth.Some methods include establishing intra-fiber crosslinks via an ionicreaction via a multivalent inorganic compound (such as aluminum sulfate)as a crosslinking agent. Some methods include establishing intra-fibercrosslinks via a radical reaction via a suitable inorganic salt (such asammonium persulfate) as a cross-linking agent. Some methods include atleast partially neutralizing the grafted polymer side chains by treatingthe grafted cellulosic material with an alkaline solution.

The materials, concepts, features, and methods briefly described aboveare clarified with reference to the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show partially schematic views of an equipment assemblyused in the In-Plane Radial Permeability (IPRP) Test described herein.

FIG. 3 shows a partially schematic view of an equipment assembly used inthe Medium Absorption Pressure (MAP) Test described herein.

FIG. 4 is a graph comparing best-fit curves showing the relationshipbetween MAP and IPRP values exhibited by example embodiments of graftedcellulose materials produced in accordance with the present disclosure(including crosslinked as well as non-crosslinked cellulose materials,collectively represented as the solid line) and example non-graftedcellulose materials (including crosslinked as well as non-crosslinkedcellulose materials, collectively represented by the dashed line).

DETAILED DESCRIPTION

The complete disclosures of the aforementioned references, and those ofall of the other references cited herein, are incorporated in theirentireties for all purposes.

There has been much research on grafting copolymers, including graftingcopolymers from cellulosic materials, such as with grafting polymerchains or “arms” consisting of acrylic acid monomers, from holocellulose(see, e.g., Okieimen, E. F., and Ebhoaye, J. E., Grafting Acrylic AcidMonomer on Cellulosic Materials, J. Macromol. Sci.-Chem., pp 349-353(1986)). However, there has been no investigation of subjecting graftedcellulose structures to conditions suitable to effect crosslinking innon-grafted cellulose materials.

The inventors have discovered, however, that various absorbent and otherproperties of certain grafted cellulose structures are changed uponundergoing crosslinking by various reactions and mechanisms. Inparticular, absorbent properties of some grafted, crosslinked cellulosestructures, such as wet bulk, absorbent capacity, permeability (e.g.,in-plane radial permeability or IPRP), capillary pressure (e.g., asmeasured by medium absorption pressure or MAP), as described in greaterdetail below, are consistent with or improved relative to those achievedby crosslinked cellulose products produced by other methods, and arefavorable when compared to cellulose fibers that are not crosslinkedand/or not grafted.

As an example of a non-crosslinked, non-grafted cellulose fiber, ableached kraft pulp product available from Weyerhaeuser NR Company underthe designation CF416 has a wet bulk of 11.71 cm³/g and an absorbentcapacity of 11.75 g/g. When grafted with polymer chains composed ofmonoethylenically unsaturated acid group-containing monomers (with agraft yield in the range of about 5-35 weight %) the resulting graftedCF416 exhibited lower wet bulk values (of about 9.8-11.2 cm³/g, withacrylic acid used as the graft species) and absorbent capacity values(of 10.0-11.3 g/g, with acrylic acid used as the graft species), butwhen subjected to subsequent crosslink treatment, the grafted,crosslinked cellulose structures produced in accordance with the presentdisclosure exhibited improved wet bulk values of about 15.0-17.0 cm³/g,and/or absorbent capacity values of about 15.0-17.0 g/g. A crosslinked(and non-grafted) fiber product available from Weyerhaeuser NR Companyunder the designation CMC530, useful as a control, has a wet bulk ofapproximately 16.4 cm³/g and an absorbent capacity of 16.5 g/g.Accordingly, the grafted, crosslinked cellulose fibers of the presentdisclosure may have suitability, for example, in absorbent applicationssimilar to those for which non-grafted crosslinked cellulose fibers areused.

As another example of this suitability, the grafted cellulose structuresproduced in accordance with the present disclosure exhibit IPRP and MAPvalues consistent with or improved relative to non-grafted cellulosefibers. As explained in greater detail below, IPRP and MAP valuesindicate a trade-off relationship that approximates a power law functionof the two properties. IPRP and MAP values relating to CMC530 and othernon-grafted controls as compared to that of example embodiments ofgrafted cellulose structures indicate that the grafted, crosslinkedstructures exhibit (or are predicted to exhibit, according to formulaeexpressing best-fit curves generated by measured IPRP and MAP data)comparable or increased values, up to 15 or 20%, or even greater.

Cellulosic fibers useful for making the grafted crosslinked cellulose ofthe present disclosure are derived primarily from wood pulp. Althoughsuitable wood pulp fibers may be obtained from chemical processes suchas the kraft and sulfite processes, with or without subsequent and/orprior mercerization and/or bleaching, the pulp fibers may also beprocessed by thermomechanical or chemithermomechanical methods, orvarious combinations thereof. Ground wood fibers, recycled or secondarywood pulp fibers, and bleached and unbleached wood pulp fibers may beused. One example starting material is prepared from long-fiberconiferous wood species, such as southern pine, Douglas fir, spruce,hemlock, and so forth. Details of the production of wood pulp fibers areknown to those skilled in the art. Suitable fibers are commerciallyavailable from a number of sources, including the Weyerhaeuser NRCompany. For example, suitable cellulose fibers produced from southernpine that may be used as the cellulose substrate in the materials of thepresent disclosure are available from the Weyerhaeuser NR Company underthe designations CF416, CF405, NF405, NB416, FR416, FR516, PW416, andPW405, amongst others.

The graft species suitable for grafting to the cellulosic fiber“backbone” to produce the grafted crosslinked cellulose materials of thepresent disclosure include those that may be described asmonoethylenically unsaturated acid group-containing monomers, whichinclude, for example, acrylic acid, methacrylic acid, crotonic acid,isocrotonic acid, maleic acid, fumaric acid, itaconic acid,vinylsulfonic acid, 2-acrylamido-2-methyl-1-propane sulfonic acid, vinylacetic acid, methallyl sulfonic acid, and so forth, as well as theiralkali and/or ammonium salts, and various combinations of theaforementioned examples. The choice of suitable graft species is guidedin part by the nature of the backbone from which the grafted arms aregrown, achieving a suitable polymer architecture, the desired end resultof the crosslink treatment, and so forth.

For example, grafting, in the context of polymer chemistry, refers ingeneral to the synthesis of polymer chains attached to a substrate, andthus encompasses mechanisms such as “grafting to,” which refers to apolymer chain adsorbing onto a substrate out of solution, as well as“grafting from,” which refers to initiating and propagating a polymerchain (such as by step-growth addition of monomer units) at a graftingsite on the substrate. The latter mechanism is generally considered tooffer greater control over the resulting polymer architecture, densityof grafting sites, polymer chain lengths and linearity, and so forth.Considering these factors, graft species that include one or more acidgroups are chemically appropriate when considered against a goal ofestablishing intra-fiber crosslinks, such as chain-to-chain crosslinksbetween grafted arms of an individual cellulose fiber. Further,monoethylenenically unsaturated graft species are suitable for thegrafted crosslinked cellulose materials herein because of their abilityto graft to a cellulose substrate without creating additional branchesor side chains (as opposed, for example, to species with more than oneunsaturated group, the use of which is more difficult to control).Monomeric graft species are considered to be easier to control, in termsof reactivity, density of polymer chains, establishing desired chainlengths and polymer chain architectures (e.g., linear, unbranchedpolymers grafted at one end to the cellulose backbone), suppressingcrosslink reactions from occurring in the grafting stage, and so forth.

“Grafting,” as the term is used herein, refers collectively to theprocesses of initiation, growth, and termination of growthpolymerization of the (monomeric) graft species from one or moregrafting sites on the cellulosic substrate. Typically, graftingaccording to methods discussed herein is performed in situ, in which aninitiator is used, such as to create active centers on the substrateand, usually to a lesser extent, initiate homopolymerization in theaqueous phase. As such, although the grafting processes described hereinproceed mainly by way of the “grafting from” mechanism described above,the term does not exclusively refer to this mechanism. Rather,“grafting” also encompasses the “grafting to” mechanism, othermechanisms, and/or combinations thereof. Moreover, the terms “to” and“from,” when used when referring to grafting, do not exclusively referto the corresponding grafting mechanism, but instead may each encompasssome degree of the other grafting mechanism (or mechanisms).

It was found that by controlling the amount of graft species used,various levels of grafting were obtained, characterized by graft yield%, defined below as the additional weight of a grafted sampleattributable to the polymerized graft species. For example, acrylic acidreadily grafted from cellulose up to approximately 30% graft yield.Additionally, by varying the levels (e.g., weight %) of initiator, thenumber of graft sites available on the cellulosic substrate was altered.Variations in graft yield were also obtained by varying the ratio ofcellulose to graft species.

In general, grafted cellulose fibers were produced in situ by dissolvinga measured amount of initiator, such as cerium(IV) sulfate, in deionizedwater, and then dissipating a designated amount of graft species, suchas 4-60 weight % acrylic acid (based on the oven-dry weight of thecellulose), in the solution. The levels of cerium(IV) sulfate initiatorwere varied between about 4.5-7.0 weight % (based on the oven-dry weightof the cellulose). The grafting solution was added to the cellulosicsubstrate, the treated cellulose was allowed to react, then washed andfiltered to remove unreacted grafting solution and excess homopolymersof the graft species, dried, and weighed to determine graft yield %. Insome examples, cellulose-graft-poly(acrylic) acid was then treated withdilute solutions of sodium hydroxide to at least partially neutralizethe grafted arms on the cellulosic substrate.

A variety of cross-linking agents and reaction mechanisms were appliedto various grafted cellulose materials, including ionic crosslinkingreactions using multivalent inorganic compounds (such as aluminumsulfate, a trivalent salt, and titanium-based crosslinking agents),covalent ester crosslinking reactions using polymeric crosslinkingagents (for example, a homopolymer of the graft species, such aspoly(acrylic) acid or “PAA” when acrylic acid was used as the graftspecies), pentaerythritol, and various hyperbranched polymers (“HPB”),and a radical-based cross-linking mechanism using ammonium persulfate.Each was performed, at various levels, on various graft yield levels offiberized, grafted cellulose and/or partially neutralized graftedcellulose, with various factors determining the conditions and amountsof reagents selected (e.g. solubility in water, ability to distributeevenly across the grafted cellulose, etc.).

In general, the grafted structures assumed a polymer architectureconsisting mainly of linear, unbranched polymer chain arms attached tothe cellulose backbone at one end. When subjected to the variouscross-linking reactions, the resulting crosslinked, grafted structuresgenerally exhibited intra-fiber crosslinks between grafted arms (such asvia pentaerythritol, or a hyperbranched polymer), also referred to aschain-to-chain crosslinks. Additionally, in some cases, for example whentreated with polymeric crosslinking agents, some of the chain armsattached to the cellulose backbone at more than one point, also referredto as chain-to-cellulose crosslinks, and some polymer bonded along thecellulose backbone. While not being bound by theory, it is believed thatthe stiffness and resiliency imparted by establishing intra-fiberchain-to-chain and/or chain-to-cellulose crosslinks strengthen the highvoid volume structure and provide the observed, improved absorbentproperties as compared to non-crosslinked grafted cellulose fibers andnon-crosslinked, non-grafted cellulose fibers.

Although the methods disclosed herein primarily establish intra-fibercrosslinks (such as chain-to-chain crosslinks between grafted arms of anindividual cellulose fiber, chain-to-cellulose crosslinks between agrafted arm of a cellulose fiber to elsewhere on the cellulose fiber,and so forth), some materials also exhibited a minor degree ofinter-fiber crosslinking, such as between separate cellulose fibersand/or grafted chains thereon. In general, inter-fiber crosslinking isthought to increase the number of “knots” in the resulting fibers.Although a higher knot content is generally not considered to be adesirable characteristic in cellulosic material used in absorbentapplications (both for the sake of overall product appearance, as wellas for the comparative ease of processing lower knot content fibers), agreater degree of inter-fiber crosslinking may be achieved by using theprocesses described herein, such as, for example, by applying and curingthe crosslinking agent to cellulose in sheet form, rather than tofiberized cellulose. Other variations may include selecting graftspecies and/or crosslinking agent(s) appropriate for establishing suchinter-fiber crosslinking, suitably varying process conditions, and soforth, to achieve a desired amount of inter-fiber crosslinking. All ofsuch variations are considered to be within the scope of thisdisclosure.

AFAQ Analysis

Some performance properties of the grafted, crosslinked cellulosiccompositions according to the present disclosure—specifically, absorbentproperties of wet bulk, wick time, wick rate, and absorbentcapacity—were determined using the Automatic Fiber Absorption Quality(AFAQ) Analyzer (Weyerhaeuser Co., Federal Way, Wash.), according to thefollowing procedure:

A dry 4-gram sample of the pulp composition is placed through a pinmillto open the pulp, and then airlaid into a tube. The tube is then placedin the AFAQ Analyzer. A plunger then descends on the airlaid fluff padat a pressure of 0.6 kPa. The pad height is measured, and the pad bulk(or volume occupied by the sample) is determined from the pad height.

The weight is increased to achieve a pressure of 2.5 kPa and the bulkrecalculated. The result is two bulk measurements on the dry fluff pulpat two different pressures.

While the dry fluff pulp is still compressed at the higher pressure,water is introduced into the bottom of the tube (to the bottom of thepad), and the time required for water to wick upward through the pad andreach the plunger (defined as wick time) is measured. The bulk of thewet pad at 2.5 kPa is also calculated. From distance measurements usedto calculate the bulk, the wick rate is determined by dividing the wicktime by the distance traveled by the water (e.g. the height of thewetted fluff pad). The plunger is then withdrawn from the tube and thewet pad is allowed to expand for 60 seconds. In general, the moreresilient the sample, the more it will expand to reach its wet reststate. Once expanded, this resiliency is measured by reapplying theplunger to the wet pad at 0.6 kPa and determining the bulk. The finalbulk of the wet pad at 0.6 kPa is considered to be the “wet bulk at 0.6kPa” (in cm³/g, indicating volume occupied by the wet pad, per weight ofthe wet pad, under the 0.6 kPa plunger load) of the pulp composition.When the term “wet bulk” is used herein, it refers to “wet bulk at 0.6kPa” as determined according to this procedure.

Absorbent capacity is calculated by weighing the wet pad after water isdrained from the equipment and reported as grams water per gram drypulp.

In-Plane Radial Permeability (IPRP) Analysis

Permeability generally refers to the quality of a porous material thatcauses it to allow liquids or gases to pass through it and, as such, isgenerally determined from the mass flow rate of a given fluid throughit. The permeability of an absorbent structure is related to thematerial's ability to quickly acquire and transport a liquid within thestructure, both of which are key features of an absorbent article.Accordingly, measuring permeability is one metric by which a material'ssuitability for use in absorbent articles may be assessed.

The following test is suitable for measurement of the In-Plane RadialPermeability (IPRP) of a porous material. The quantity of a salinesolution (0.9% NaCl) flowing radially through an annular sample of thematerial under constant pressure is measured as a function of time.

Testing is performed at 23° C.±2 C.° and a relative humidity 50%±5%. Allsamples are conditioned in this environment for twenty four (24) hoursbefore testing.

The IPRP sample holder 400 is shown in FIG. 1 and comprises acylindrical bottom plate 405, top plate 410, and cylindrical stainlesssteel weight 415.

Top plate 410 comprises an annular base plate 420 9 mm thick with anouter diameter of 70 mm and a tube 425 of 150 mm length fixed at thecenter thereof. The tube 425 has an outer diameter of 15.8 mm and aninner diameter of 12 mm. The tube is adhesively fixed into a circular 16mm hole in the center of the base plate 420 such that the lower edge ofthe tube is flush with the lower surface of the base plate, as depictedin FIG. 1. The bottom plate 405 and top plate 410 are fabricated fromLexan® or equivalent. The stainless steel weight 415 has an outerdiameter of 70 mm and an inner diameter of 15.9 mm so that the weight isa close sliding fit on tube 425. The thickness of the stainless steelweight 415 is approximately 22 mm and is adjusted so that the totalweight of the top plate 410 and the stainless steel weight 415 is 687g±1 g to provide 2.0 kPa of confining pressure during the measurement.

Bottom plate 405 is approximately 25 mm thick and has two registrationgrooves 430 cut into the lower surface of the plate such that eachgroove spans the diameter of the bottom plate and the grooves areperpendicular to each other. Each groove is 1.5 mm wide and 2 mm deep.Bottom plate 405 has a horizontal hole 435 which spans the diameter ofthe plate. The horizontal hole 435 has a diameter of 8 mm and itscentral axis is 15 mm below the upper surface of bottom plate 405.Bottom plate 405 also has a central vertical hole 440 which has adiameter of 8 mm and is 10 mm deep. The central hole 440 connects to thehorizontal hole 435 to form a T-shaped cavity in the bottom plate 405.The outer portions of the horizontal hole 435 are threaded toaccommodate pipe elbows 445 which are attached to the bottom plate 405in a watertight fashion. One elbow is connected to a verticaltransparent tube 460 with a total height of 175 mm measured from thebottom of bottom plate 405 (including elbow 445) and an internaldiameter of 6 mm. The tube 460 is scribed with a suitable mark 470 at aheight of 100 mm above the upper surface of the bottom plate 420. Thisis the reference for the fluid level to be maintained during themeasurement. The other elbow 445 is connected to the fluid deliveryreservoir 700 (described below) via a flexible tube.

A suitable fluid delivery reservoir 700 is shown in FIG. 2. Reservoir700 is situated on a suitable laboratory jack 705 and has an air-tightstoppered opening 710 to facilitate filling of the reservoir with fluid.An open-ended glass tube 715 having an inner diameter of 10 mm extendsthrough a port 720 in the top of the reservoir such that there is anairtight seal between the outside of the tube and the reservoir.Reservoir 700 is provided with an L-shaped delivery tube 725 having aninlet 730 that is below the surface of the fluid in the reservoir, astopcock 735, and an outlet 740. The outlet 740 is connected to elbow445 via flexible plastic tubing 450 (e.g. Tygon®). The internal diameterof the delivery tube 725, stopcock 735, and flexible plastic tubing 450enable fluid delivery to the IPRP sample holder 400 at a high enoughflow rate to maintain the level of fluid in tube 460 at the scribed mark470 at all times during the measurement. The reservoir 700 has acapacity of approximately 6 liters, although larger reservoirs may berequired depending on the sample thickness and permeability. Other fluiddelivery systems may be employed provided that they are able to deliverthe fluid to the sample holder 400 and maintain the level of fluid intube 460 at the scribed mark 470 for the duration of the measurement.

The IPRP catchment funnel 500 is shown in FIG. 2 and comprises an outerhousing 505 with an internal diameter at the upper edge of the funnel ofapproximately 125 mm. Funnel 500 is constructed such that liquid fallinginto the funnel drains rapidly and freely from spout 515. A stand withhorizontal flange 520 around the funnel 500 facilitates mounting thefunnel in a horizontal position. Two integral vertical internal ribs 510span the internal diameter of the funnel and are perpendicular to eachother. Each rib 510 is 1.5 mm wide and the top surfaces of the ribs liein a horizontal plane. The funnel housing 500 and ribs 510 arefabricated from a suitably rigid material such as Lexan® or equivalentin order to support sample holder 400. To facilitate loading of thesample it is advantageous for the height of the ribs to be sufficient toallow the upper surface of the bottom plate 405 to lie above the funnelflange 520 when the bottom plate 405 is located on ribs 510. A bridge530 is attached to flange 520 in order to mount two digital calipers 535to measure the relative height of the stainless steel weight 415. Thedigital calipers 535 have a resolution of ±0.01 mm over a range of 25mm. A suitable digital caliper is a Mitutoyo model 543-492B orequivalent. Each caliper is interfaced with a computer to allow heightreadings to be recorded periodically and stored electronically on thecomputer. Bridge 530 has a circular hole 22 mm in diameter toaccommodate tube 425 without the tube touching the bridge.

Funnel 500 is mounted over an electronic balance 600, as shown in FIG.2. The balance has a resolution of ±0.01 g and a capacity of at least1000 g. The balance 600 is also interfaced with a computer to allow thebalance reading to be recorded periodically and stored electronically onthe computer. A suitable balance is Mettler-Toledo model MS6002S orequivalent. A collection container 610 is situated on the balance pan sothat liquid draining from the funnel spout 515 falls directly into thecontainer 610.

The funnel 500 is mounted so that the upper surfaces of ribs 510 lie ina horizontal plane. Balance 600 and container 610 are positioned underthe funnel 500 so that liquid draining from the funnel spout 515 fallsdirectly into the container 610. The IPRP sample holder 400 is situatedcentrally in the funnel 500 with the ribs 510 located in grooves 430.The upper surface of the bottom plate 405 must be perfectly flat andlevel. The top plate 410 is aligned with and rests on the bottom plate405. The stainless steel weight 415 surrounds the tube 425 and rests onthe top plate 410. Tube 425 extends vertically through the central holein the bridge 530. Both calipers 535 are mounted firmly to the bridge530 with the foot resting on a point on the upper surface of thestainless steel weight 415. The calipers are set to zero in this state.The reservoir 700 is filled with 0.9% saline solution and re-sealed. Theoutlet 740 is connected to elbow 445 via flexible plastic tubing 450.

An annular sample 475 of the material to be tested is cut by suitablemeans. The sample has an outer diameter of 70 mm and an inner holediameter of 12 mm. One suitable means of cutting the sample is to use adie cutter with sharp concentric blades.

The top plate 410 is lifted enough to insert the sample 475 between thetop plate and the bottom plate 405 with the sample centered on thebottom plate and the plates aligned. The stopcock 735 is opened and thelevel of fluid in tube 460 is set to the scribed mark 470 by adjustingthe height of the reservoir 700 using the jack 705 and by adjusting theposition of the tube 715 in the reservoir. When the fluid level in thetube 460 is stable at the scribed mark 470 initiate recording data fromthe balance and calipers by the computer. Balance readings and timeelapsed are recorded every 10 seconds for five minutes. The averagesample thickness B is calculated from all caliper reading between 60seconds and 300 seconds and expressed in cm. The flow rate in grams persecond is the slope calculated by linear least squares regression fit ofthe balance reading (dependent variable) at different times (independentvariable) considering only the readings between 60 seconds and 300seconds.

Permeability k is then calculated by the following equation:

$\begin{matrix}{k = \frac{\left( {Q/\rho_{1}} \right) \cdot \mu \cdot {\ln \left( {R_{0}/R_{i}} \right)}}{2{\pi \cdot B \cdot \Delta}\; p}} & (1)\end{matrix}$

Where:

k is the permeability (cm²);

Q is the flow rate (g/s);

ρ₁ is the liquid density at 20° C. (g/cm³);

μ is the liquid viscosity at 20° C. (Pa·s);

R₀ is the outer sample radius (cm);

R_(i) is the inner sample radius (cm);

B is the average sample thickness (cm); and

Δp is the pressure drop (Pa) calculated according to the followingequation:

$\begin{matrix}{{\Delta \; p} = {\left( {{\Delta \; h} - \frac{B}{2}} \right) \cdot g \cdot \rho_{1} \cdot 10}} & (2)\end{matrix}$

Where:

Δh is the measured liquid hydrostatic pressure (cm);

g is the acceleration constant (m/sec²); and

ρ₁ is the liquid density (g/cm³).

In-plane radial permeability is dependent on the fluid being used, sothe IPRP value (in cm²/MPa·sec) may be defined and calculated asfollows:

IPRP value=(k/μ)  (3)

Where:

k is the permeability (cm²); and

μ is the liquid viscosity at 20° C. (MPa·s).

MAP Analysis

Capillary pressure can be considered representative of a material'sability to wick fluid by capillary action and is expressed in thecontext of the present disclosure in terms of Medium Absorption Pressure(MAP), as explained below.

Capillary pressure measurements are made on a TRI/Autoporosimeter(TRI/Princeton Inc. of Princeton, N.J.). The TRI/Autoporosimeter is anautomated computer-controlled instrument for measuring capillarypressure in porous materials, which can be schematically represented inFIG. 3. Complimentary Automated Instrument Software, Release Version2007.6WD, is used to capture the data. More information on theTRI/Autoporosimeter, its operation and data treatments can be found inThe Journal of Colloid and Interface Science 162 (1994), pp. 163-170,incorporated here by reference.

As used herein, determining capillary pressure hysteresis curve of amaterial as function of saturation, involves recording the increment ofliquid that enters a porous material as the surrounding air pressurechanges. A sample in the test chamber is exposed to precisely controlledchanges in air pressure which at equilibrium (no more liquiduptake/release) corresponds to the capillary pressure.

The equipment operates by changing the test chamber air pressure inuser-specified increments, either by decreasing pressure (increasingpore size) to absorb liquid, or increasing pressure (decreasing poresize) to drain liquid. The liquid volume absorbed (or drained) ismeasured with a balance at each pressure increment. The saturation isautomatically calculated from the cumulative volume.

All testing is performed at 23° C.±2 C.° and a relative humidity of50%±5%. A saline solution of 0.9% weight to volume in deionized water isused. The surface tension (mN/m), contact angle (°), and density (g/cc)for all solutions are determined by any method known in the art.Alternatively (as done for measuring the Examples below), referencevalues for these parameters may be provided to the TRI/Autoporosimeter'ssoftware.

Surface tension (mN/m), contact angle (°), and density (g/cm³) isprovided to the instrument's software. Reference values used for thetests described herein were as follows: surface tension of 72 mN/m;contact angle of 0°; and liquid density of 1 g/cm³. The balance isleveled at 156.7 g and equilibration rate set to 90 mg/min. The poreradius protocol (corresponding to capillary pressure steps) scanscapillary pressures according to the following equation:

R=2γ cos θ/Δp  (4)

Where:

R is the pore radius;

γ is the surface tension;

θ is the contact angle; and

Δp is the capillary pressure.

Tests are performed with the sample compressed with an applied load ofapproximately 0.3 psi. The weight applied to the sample is 428 g and is50 mm in diameter.

The pressure sequence in Table 1, below, is applied to the measurementcell in the standard test protocol which corresponds to an individualpore radius as indicated.

TABLE 1 Height (mm) Radius (μm) 1 600 24.5 2 450 32.7 3 350 42.0 4 30049.0 5 250 58.8 6 200 73.5 7 150 98.0 8 100 147 9 80 184 10 60 245 11 40368 12 20 735 13 10 1470 14 20 735 15 40 368 16 60 245 17 80 184 18 100147 19 150 98.0 20 200 73.5 21 250 58.8 22 300 49.0 23 350 42.0 24 45032.7 25 600 24.5

The sample is cut into a circle with 5 cm diameter and then conditionedat 23° C.±2 C.° and a relative humidity 50%±5% for at least 24 hoursbefore testing. The sample weight (to ±0.001 g) is measured. The emptysample chamber is closed. After the instrument has applied theappropriate air pressure to the cell, the liquid valve is closed and thechamber is opened. The specimen and confining weight are placed into thechamber and the chamber is closed. After the instrument has applied theappropriate air pressure to the cell, the liquid valve is opened toallow free movement of liquid to the balance and the test under theradius protocol is started. The instrument proceeds through oneabsorption/desorption cycle (also called a hysteresis loop). A blank(without specimen) is run in like fashion.

For calculations and reporting, the mass uptake from a blank run isdirectly subtracted from the uptake of the sample. Medium AbsorptionPressure (MAP) is the pressure at which 50% of the liquid uptake hasbeen achieved—or, in other words, the pressure that corresponds to 50%of the total liquid absorbed on the absorption branch of the hysteresisloop generated by the autoporosimeter.

The following examples summarize representative methods of treatingcellulose fibers in accordance with the methods and concepts discussedabove, and are illustrative in nature. The reagent amounts, times,conditions, and other process conditions may be varied from thosedisclosed in the specific representative procedures disclosed in thefollowing examples without departing from the scope of the presentdisclosure.

Example 1 Grafting

Representative procedure: cellulose pulpsheets (CF416, 93% solids,Columbus Mill, from Weyerhaeuser NR Company, Federal Way, Wash.) werecut into rectangles (of 13.25″×4″), weighed, and placed into re-sealableplastic bags in pairs. A Ce⁴⁺ catalyst solution was produced by stirringand dissolving a measured quantity of ammonium cerium(IV) sulfate (94%,from Sigma Aldrich) in 150.0 mL deionized water. Acrylic acid (99%, with180-200 ppm MEHQ inhibitor, from Sigma Aldrich) in a measured volume wasthen added to the Ce⁴⁺ solution and stirred for 5 minutes. The resultingsolution was slowly poured over the cellulose pulpsheets, on both sides,in the bag, which was then sealed and allowed to equilibrate at roomtemperature overnight.

The sealed bag was then cured in a ventilated oven at 50° C. for 2hours, followed by cooling to room temperature. The treated cellulose(cellulose-graft-poly(acrylic) acid) was then washed with 2.5 Ldeionized water in a Waring Blendor at low speed. Unreacted graftingsolution, excess homopolymers of poly(acrylic) acid, and otherimpurities, were removed via vacuum filtration with Buchner funnel andfilter paper, washed and vacuum filtered again, then oven-driedovernight at 50° C.

Graft yield was calculated using the following formula:

% Graft Yield=[(W ₂ −W ₁)/W ₁]×100  (5)

Where W₁=weight of starting cellulose material, and W₂=grafted productweight.

Representative data indicating weights and volumes used in a number ofruns performed according to Example 1, and graft yields achieved, areshown in Table 2.

TABLE 2 Wt Starting Material AA Catalyst Wt Product Graft Yield Run #(g) (mL) (g) (g) (%) 1 48.79 2.2 3.44 51.06 4.66 2 48.62 15.5 2.92 57.6018.47 3 48.55 9.0 2.92 54.40 12.06 4 48.57 2.2 3.44 50.74 4.46 5 48.269.0 2.92 53.82 11.53 6 48.64 2.7 2.40 50.62 4.07 7 48.37 9.0 2.92 53.7311.08 8 48.65 9.0 2.92 54.44 11.91 9 48.87 16.0 2.40 58.04 18.76 1048.74 10.0 2.40 54.33 11.47 11 48.72 9.0 2.92 54.33 11.51 12 48.18 2.72.40 50.35 4.50 13 48.44 8.5 3.44 54.24 11.97 14 48.75 2.5 2.92 50.834.27 15 48.21 14.0 3.44 56.49 17.17 16 (control) 48.92 0 0 49.04 0.25

A variety of cross-linking agents and reaction mechanisms were thenapplied to cellulose-graft-poly(acrylic) acid materials prepared inaccordance with the procedure in Example 1, as described in Examples 2-7infra.

Example 2 PAA Ester Cross-Linking

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, 12″×12″ Britishhandsheets were prepared using all the material from one run (there wassome loss during process). The handsheets were equilibrated to 93%solids in a humidity-controlled room. Each handsheet was cut into strips(of 12″×4″).

Polyacrylic acid (“PAA”) crosslinking agent (Aquaset™ 1676 availablefrom The Dow Chemical Company; other suitable examples of suitablecrosslinking agents are listed in U.S. Pat. App. Pub. No. 20110077354 ofStoyanov, et al.) was applied, in some cases in the presence of a sodiumhypophosphite (“SHP”) catalyst, to the handsheet strips. The treatedstrips were allowed to equilibrate, then air-dried, fiberized with aKamas hammermill, and cured.

Representative data indicating weights and volumes used are shown inTable 3.

TABLE 3 Wt Handsheet @93% PAA PAA Run (g) % (g) 1 53.05 0 0 2 59.16 1.51.49 3 56.40 0 0 4 52.78 3 2.96 5 55.70 1.5 1.47 6 52.68 0 0 7 55.58 1.51.48 8 56.38 3 2.97 9 60.01 0 0 10 56.31 1.5 1.49 11 56.34 1.5 1.49 1252.37 3 2.94 13 56.19 1.5 1.48 14 52.86 1.5 1.49 15 58.35 0 0 16(control) 51.31 6.5 6.44

Example 3 Pentaerythritol Ester Cross-Linking

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, an aqueous slurry wasprepared using all the material from one run, which was thenvacuum-filtered using a Buchner funnel to produce a pad. The pad wasthen oven-dried at 50° C. to constant weight.

A solution of 5.00 g pentaerythritol (from Sigma Aldrich) and 0.15 gsodium hypophosphite in 60 mL deionized water was prepared at 40° C.,which was then cooled to room temperature and evenly applied to bothsides of the pad via transfer pipette, and the treated pad was placed ina sealed plastic bag and allowed to equilibrate at room temperatureovernight.

The pad was then fiberized in a Waring Blendor, and cured in an oven at193° C. for 5 minutes.

Example 4 Trivalent Salt Ionic Cross-Linking

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, an aqueous slurry wasprepared using all the material from one run, which was thenvacuum-filtered using a Buchner funnel to produce a pad. The pad wasthen air-dried to 43.5% solids content.

An aluminum sulfate solution was prepared by dissolving 10.00 g ofaluminum sulfate octodecahydrate (from Sigma Aldrich) in 250 mLdeionized water. The solution was evenly applied to both sides of theair-dried pad via transfer pipette, and the treated pad was placed in asealed plastic bag and allowed to equilibrate at room temperatureovernight.

The treated pad was then vacuum filtered in a Buchner funnel and gentlyrinsed, once, with 500 mL deionized water, then air-dried at roomtemperature until constant weight was achieved.

Example 5 Titanium-Based Ionic Cross-Linking

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, an aqueous slurry wasprepared using all the material from one run, which was thenvacuum-filtered using a Buchner funnel to produce a pad. The pad wasthen oven-dried to constant weight.

A solution was prepared by dissolving 6.61 g of Tyzor® LA (lactic acidtitanate chelate, from DuPont) in 26.4 mL deionized water at roomtemperature. The solution was evenly applied to both sides of theair-dried pad via transfer pipette, and the treated pad was placed in asealed plastic bag and allowed to equilibrate at room temperatureovernight.

The pad was then fiberized in a Waring Blendor, and cured in an oven at175° C. for 15 minutes.

Example 6 Radical Cross-Linking

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, 12″×12″ Britishhandsheets were prepared using 50 g of the material. The handsheets wereequilibrated to 93% solids in a humidity-controlled room. Each handsheetwas cut into strips (of 12″×4″).

A solution of 2.22 g of ammonium persulfate (from Sigma Aldrich) in 45mL deionized water was prepared and evenly applied across the handsheetstrips via transfer pipette, and the treated strips were placed in asealed plastic bag and allowed to equilibrate at room temperatureovernight.

The treated strips were then air-dried to about 70% solids and thenfiberized with a Kamas hammermill. The material was then gently andevenly sprayed with 110 mL deionized water, placed in a foil pouch thatwas perforated to allow evaporation, and oven-cured at 390° F. (199° C.)for 15 minutes. After removal from the oven, the pouch was cooled toroom temperature, and then the material was removed and allowed toair-dry at room temperature until constant weight was achieved.

Example 7 Ester Cross-Linking with Hyperbranched Polymers

Representative procedure: using cellulose-graft-poly(acrylic) acidprepared according to the procedure in Example 1, 12″×12″ Britishhandsheets were prepared using 50 g of the material. The handsheets wereequilibrated to 93% solids in a humidity-controlled room. Each handsheetwas cut into strips (of 12″×4″).

A solution of a measured quantity of a hyperbranched polymer (e.g., 0.22g Lutensit® Z96 or 0.95 Lutensol® FP620, both from BASF) along with 0.14g sodium hypophosphite in 45 mL deionized water was prepared and evenlyapplied across the handsheet strips via transfer pipette, and thetreated strips were placed in a sealed plastic bag and allowed toequilibrate at room temperature overnight.

The treated strips were then air-dried to about 70% solids, fiberizedwith a Kamas hammermill, air-dried overnight, and cured at 370° F.(187.8° C.) for 5 minutes in a large dispatch oven.

AFAQ analysis was performed on the cellulose-graft-poly(acrylic) acidmaterials prepared in accordance the representative procedure of Example1, as well as on various crosslinked cellulose-graft-poly(acrylic) acidmaterials prepared in accordance with the representative procedures ofExamples 2-7. The representative values presented in Table 4 (below) areaverages from multiple runs of the indicated crosslink method on theindicated graft yield % level of cellulose-graft-poly(acrylic) acid.

TABLE 4 AFAQ Analysis Wick Wick Wet Bulk Absorbent Time Rate 0.6 kPaCapacity Sample Description Sample # sec mm/s cm³/g g/g % graftcrosslink method 1 2.1 9.42 9.73 9.93 11 ionic (Al³⁺) 2 2.6 8.03 12.0712.49 11 ester (pentaerythritol) 3 2.0 10.58 10.55 10.62 11 titanium(Tyzor LA) 4 4.6 6.63 16.53 16.74 12 ester (PAA) 5 4.0 6.89 11.19 11.326.6 none 6 3.8 8.43 15.92 15.83 6.6 ester (HPB) 7 3.8 7.12 10.93 11.0111 none 8 3.7 6.32 11.26 11.56 11 radical 9 3.3 9.11 16.73 16.43 11ester (HPB) 10 3.4 7.22 10.12 10.17 19.8 none 11 2.9 9.51 16.15 15.7519.8 ester (HPB) 12 3.2 6.82 9.84 9.97 25.2 none 13 2.85 8.72 15.5315.09 25.2 ester (HPB) Control A (CMC530) 2.3 11.87 16.39 16.45 n/a n/aControl B (CMC530) 2.6 10.71 16.45 16.65 n/a n/a Control C (CF416)  — —11.71 11.75 n/a n/a

Table 4 shows, for example, that cellulose-graft-poly(acrylic) acidproduced from CF416 cellulose pulp fiber tends to exhibit lower wet bulkand absorbent capacity values as compared with untreated (i.e.non-grafted, non-crosslinked) CF416, across a range of graft yieldlevels tested. However, when subjected to subsequent crosslinktreatment, the grafted, crosslinked cellulose structures producedthereby generally exhibited improved wet bulk values and/or absorbentcapacity values. For example, ester cross-linking using PAA or ahyperbranched polymer as the cross-linking agent yielded improvements inwet bulk and absorbent capacity values, from about 6% to over 40%, ascompared to untreated cellulose. Indeed, the improved values of theseproperties were found to be comparable with those exhibited bynon-grafted, crosslinked cellulose samples (e.g., CMC530). Somecrosslinking treatments, however, such as ionic trivalent salt andtitanium-based reactions, did not increase, and in some cases furtherdecreased, wet bulk and absorbent capacity values as compared tountreated cellulose. Also, in general, cellulose-graft-poly(acrylic)acid materials produced from CF416 exhibited lower wick rates thannon-grafted, crosslinked cellulose samples, but when subjected tosubsequent crosslink treatment, wick rates were seen to further decreasewith some PAA/SHP and radical treatment processes, but increase withothers (such as with ester cross-linking with HPB and pentaerythritol,and ionic processes). Thus, various absorbent properties may be modifiedthrough a selection of graft species, cross-linking reaction, and otherprocess conditions.

Another performance metric by which the grafted, crosslinked cellulosematerials produced in accordance with the present disclosure may becharacterized and/or compared is by means of liquid permeability andcapillary pressure, two properties important for absorbent products.Liquid permeability may be measured by in-plane radial permeability(IPRP) and capillary pressure may be measured by medium absorptionpressure (MAP), according to the tests described above.

As noted above, there is a trade-off between IPRP and MAP with knownabsorbent materials, including cellulose materials, synthetic fibers,blends, and so forth, in which IPRP tends to decrease as MAP increases.This trade-off is illustrated, for example, in FIG. 4, in the form of adashed line following the least-squares best-fit curve that correspondsto IPRP and MAP values exhibited by example cellulose fiber controls,including crosslinked cellulose products such as CMC530 (used as acontrol in the Examples and subsequent AFAQ analysis described above) aswell as non-crosslinked cellulose products such as NB416, CF416, and soforth. The IPRP and MAP values are shown in Table 5, below. The curvecan be described mathematically as a power law function y=mx^(z), withIPRP value as the abscissa and MAP as the ordinate. For the Table 5data, the best-fit curve for the non-grafted controls can be expressedby the formula y=896.38x^(−0.549), with R²=0.9479.

TABLE 5 IPRP Value MAP (cm²/MPa · sec) (cm H₂O) Material 594 21.3non-crosslinked 651 23.9 control 736 23.7 737 25.6 759 31.6 2144 12.9crosslinked control 2360 12.3 2674 11.8 2723 11.5 3061 11.6 3219 10.33453 9.7 3659 10.6 3708 9.8 4350 9.0 5189 8.1 5296 8.0

As exemplified, for example, in FIG. 4 (and Table 5), cellulose fibershave been observed to be bounded by a maximum IPRP value of about 5400cm²/MPa·sec and a maximum MAP value of about 32 cm H₂O. Higher IPRPvalues have been achieved, but only with blends of cellulose withsynthetic fibers (e.g. polyethylene, polypropylene and/or polyesterfibers) or synthetic nonwovens produced from, for example, polyethylene,polypropylene and/or polyester fibers or filaments.

Focusing in particular on crosslinked cellulose fibers, such productshave been observed to be bounded by a maximum MAP value of about 13 cmH₂O.

The grafted, crosslinked cellulose materials prepared in accordance withthe present disclosure, however, exhibit IPRP values as high as 7700cm²/MPa·sec and MAP values up to 20 cm H₂O.

In some examples, IPRP and MAP values for grafted, crosslinked cellulosematerials approximate a trade-off curve that is slightly shifted (i.e.raised) and also elongated (i.e. spans a broader IPRP range), withrespect to that exhibited by the non-grafted cellulose controls, asshown in FIG. 4. The trade-off for the grafted materials is shown as asolid line following the best-fit curve for the example data presentedin Table 6, below.

TABLE 6 IPRP Value MAP (cm²/MPa · sec) (cm H₂O) Material 517 27.67grafted, non- 601 37.55 crosslinked 1113 19.13 grafted, 1139 20.02crosslinked 1143 18.06 1267 18.31 1465 17.56 1479 17.01 1555 16.62 177416.45 1868 14.79 2122 13.44 2129 13.09 2139 13.58 2152 13.34 2270 13.172270 13.20 2308 13.31 2384 13.13 2386 13.16 2470 12.95 2502 12.95 253212.94 2564 12.69 2649 12.55 2651 12.45 2710 11.78 2714 12.33 2722 12.692801 12.18 2832 11.80 2851 12.36 2875 12.15 2879 11.98 2940 12.20 297111.16 3816 9.39 4143 9.34 4434 10.07 5038 8.00 5483 9.24 5713 8.49 59999.61 7672 7.55

The best-fit curve generated by the example data set in Table 6,corresponding to grafted cellulose materials, can be expressed by theformula y=869.93x^(−0.538) (with R²=0.9471). Best-fit curves for examplematerials prepared in accordance with the present disclosure can becharacterized by the same general power law function represented by theformula y=mx^(z), with m values ranging from about 600 to about 1200(and more particularly from about 800 to about 1100), and z valuesranging from about −0.590 to −0.515 (and more particularly from about−0.560 to about −0.520). These best-fit curve models for the graftedmaterials of the present disclosure correspond to or predict the IPRPvalue for a given MAP value (and vice versa) within about +/−30% of thevalue of x (or y) in the respective formula, particularly at IPRP valuesy (in cm²/MPa·sec) ranging from about 1000 to about 7700.

Comparing the best-fit curves for the example non-grafted controls(dashed line) and the example grafted materials (solid line), the“shift” visible in FIG. 4 (and shown by the data in Tables 5 and 6)illustrates that, in the range of IPRP values exhibited by non-graftedcellulose fiber control materials (that is, a range of from about 600 toabout 5400 cm²/MPa·sec), the example grafted materials exhibit (or arepredicted to exhibit) MAP values equal to or higher than thecorresponding MAP values possessed by non-grafted cellulose materials.Also, for a given MAP value (in cm H₂O) in a range of from about 7.0 toabout 25, the grafted materials exhibit (or are predicted to exhibit)IPRP values equal to or higher than the corresponding IPRP valuespossessed by non-grafted cellulose fiber. Focusing specifically on thedata corresponding to crosslinked materials, the difference in IPRPvalues is generally up to about 20% higher for the example graftedmaterials as compared to the example non-grafted controls over an MAPrange of about 7.0 to 20 (although the differences are even greater insome instances, for example with IPRP values exhibited at MAP valuesbetween about 7.0 and about 10), and the difference in MAP values isgenerally up to about 15% higher for grafted materials over a range ofIPRP values of about 800 to 5400 cm²/MPa·sec.

Comparing the best-fit curves for the example non-grafted controls andthe example grafted materials, the “elongation” visible in FIG. 4 (andshown by the data in Tables 5 and 6) illustrates that IPRP valuesgreater than those achieved with non-grafted cellulose products (e.g.,IPRP values greater than about 5400 cm²/MPa·sec) are exhibited by thegrafted, crosslinked cellulose materials of the present disclosure.

Accordingly, the grafted, crosslinked cellulose of the presentdisclosure may have suitability, for example, in absorbent applicationssimilar to those for which non-grafted, crosslinked cellulose fibers areused, as well as other applications.

Although the inventive subject matter for which protection is sought isdefined in the appended claims, other illustrative, non-exclusiveexamples of inventive subject matter according to the present disclosureare described in the following enumerated paragraphs:

A. A cellulosic material comprising a cellulose fiber and polymer chainscomposed of at least one monoethylenically unsaturated acidgroup-containing monomer grafted thereto, wherein one or more of saidcellulose fiber and said polymer chains are crosslinked.

A.1. The material of A, wherein the cellulose fiber is wood fiber.

A.2. The material of A or A.1, wherein the material includes mainlyintra-fiber crosslinks.

A.3. The material of any of A through A.2, wherein the at least onemonoethylenically unsaturated acid group-containing monomer includes oneor more of acrylic acid, maleic acid, and methacrylic acid.

A.4. The material of any of A through A.3, wherein the at least onemonoethylenically unsaturated acid group-containing monomer is acrylicacid.

A.5. The material of any of A through A.4, characterized by a graftyield of 5-35 weight %.

A.6. The material of any of A through A.5, characterized by a graftyield of 10-20 weight %.

A.7. The material of any of A through A.6, wherein the material has awet bulk at least 6% greater than untreated cellulose fiber.

A.8. The material of any of A through A.7, wherein the material has awet bulk at least 40% greater than untreated cellulose fiber.

A.9. The material of any of A through A.8, wherein the material has awet bulk of about 10.0-17.0 cm³/g.

A.10. The material of any of A through A.9, wherein the material has awet bulk of about 15.0-17.0 cm³/g.

A.11. The material of any of A through A.10, wherein the material has anabsorbent capacity of about 10.0-17.0 g/g.

A.12. The material of any of A through A.11, wherein the material has anabsorbent capacity of about 15.0-17.0 g/g.

A.13. The material of any of A through A.12, wherein the material has anIPRP value of about 1000 to 7700 cm²/MPa·sec and a MAP of about 7.0 to20 cm H₂O.

A.14. The material of any of A through A.13, wherein for a given IPRPvalue y (in cm²/MPa·sec) from 1000 to 7700, the MAP value of thematerial (in cm H₂O) is within +/−30% of the value of x in the formulay=mx^(z); wherein m is from 600 to 1200, and wherein z is from −0.590 to−0.515.

A.15 The material of A.14, wherein z is from −0.560 to −0.520.

A.16. The material of A.14 or A.15, wherein m is from 800 to 1100.

A.17. The material of any of A through A.16, characterized in that at agiven IPRP value (in cm²/MPa·sec) from 800 to 5400, the material has aMAP value that is equal to or higher than the corresponding MAP valuepossessed by non-grafted, crosslinked cellulose fiber.

A.18. The material of A.17, wherein the material at the given IPRP valuehas a MAP value that is between 0 and 20% higher than the correspondingMAP value possessed by non-grafted, crosslinked cellulose fiber.

A.19. The material of any of A through A.17, characterized in that at agiven MAP value (in cm H₂O) from 7.0 to 20, the material has an IPRPvalue that is equal to or higher than the corresponding IPRP valuepossessed by non-grafted, crosslinked cellulose fiber.

A.20. The material of A.19, wherein the material at the given MAP valuehas an IPRP value that is between 0 and 15% higher than thecorresponding IPRP value possessed by non-grafted, crosslinked cellulosefiber.

A.21. The material of any of A through A.12, wherein the material has anIPRP value of 5400 cm²/MPa·sec or above.

B. A fibrous cellulosic material comprisingcellulose-graft-poly(acrylic) acid having a graft yield of 10-20 weight% and wherein two or more grafted polymer chains of poly(acrylic) acidare intra-fiber crosslinked.

B.1. The material of B, wherein the material has a wet bulk of about15.0-17.0 cm³/g.

B.2. The material of B or B.1, wherein said two or more polymer chainsof poly(acrylic) acid are intra-fiber crosslinked by a hyperbranchedpolymer.

B.3. The material of any of B through B.2, wherein said two or morepolymer chains of poly(acrylic) acid are intra-fiber crosslinked bypentaerythritol.

C. A method of producing a grafted, crosslinked cellulosic material, themethod comprising (a) grafting polymer chains of at least onemonoethylenically unsaturated acid group-containing monomer from acellulosic substrate to produce a grafted cellulosic material, and (b)subsequently crosslinking the grafted cellulosic material by treatingthe material with a crosslinking agent adapted to effect crosslinking ofone or more of the cellulosic substrate or the polymer chains.

C.1. The method of C, wherein the cellulosic material is cellulosefiber.

C.2. The method of C of C.1, wherein the grafting is performed in situ.

C.3. The method of any of C through C.2, wherein the grafting includesreacting the at least one monomer with the cellulosic substrate in thepresence of a grafting initiator.

C.3.1. The method of C.3., wherein the grafting initiator includesammonium cerium(IV) sulfate.

C.3.2. The method of C or C.3.1., wherein the grafting includes varyingone or more of the weight percent of the initiator, and the ratio of thecellulosic substrate to the monomer, to achieve a desired graft yieldlevel.

C.4. The method of any of C through C.3.2., including using acrylic acidas the at least one monomer.

C.5. The method of any of C through C.4., wherein the crosslinkingincludes establishing intra-fiber crosslinks via an esterificationreaction, and wherein the crosslinking agent includes one or more ofpentaerythritol, a homopolymer formed of the at least onemonoethylenically unsaturated acid group-containing monomer, and ahyperbranched polymer.

C.5.1. The method of C.5., wherein the crosslinking also includesestablishing intra-fiber chain-to-cellulose crosslinks.

C.6. The method of any of C through C.5.1, wherein the crosslinkingincludes establishing intra-fiber chain-to-chain crosslinks via an ionicreaction, and wherein the crosslinking agent includes a multivalentinorganic compound.

C.6.1. The method of C.6, wherein the crosslinking agent includesaluminum sulfate.

C.7. The method of any of C through C.5.1., wherein the crosslinkingincludes establishing intra-fiber chain-to-chain crosslinks via aradical reaction, and wherein the crosslinking agent includes aninorganic salt of a strong acid and a weak base.

C.7.1. The method of C.7, wherein the crosslinking agent includesammonium persulfate.

C.8. The method of any of C through C.7.1., further including, prior tothe crosslinking, at least partially neutralizing the grafted polymerside chains by treating the grafted cellulosic material with an alkalinesolution.

C.9. The material produced by the method of any of C through C.8.

Although the present invention has been shown and described withreference to the foregoing principles and illustrated examples andembodiments, it will be apparent to those skilled in the art thatvarious changes in form, detail, conditions, and so forth, may be madewithout departing from the spirit and scope of the invention. Thepresent invention is intended to embrace all such alternatives,modifications and variances that fall within the scope of the appendedclaims.

What is claimed is:
 1. A cellulosic material comprising a cellulosefiber and polymer chains composed of at least one monoethylenicallyunsaturated acid group-containing monomer grafted thereto, wherein oneor more of said cellulose fiber and said polymer chains are intra-fibercrosslinked.
 2. The material of claim 1, wherein the at least onemonoethylenically unsaturated acid group-containing monomer includesacrylic acid.
 3. The material of claim 1, characterized by a graft yieldof 5-35 weight %.
 4. The material of claim 3, characterized by a graftyield of 10-20 weight %.
 5. The material of claim 1, wherein thematerial has a wet bulk at least 6% greater than untreated cellulosefiber.
 6. The material of claim 1, wherein the material has a wet bulkof about 10.0-17.0 cm³/g.
 7. The material of claim 1, wherein thematerial has an IPRP value of about 1000 to 7700 cm²/MPa·sec and a MAPof about 7.0 to 20 cm H₂O.
 8. The material of claim 1, wherein for agiven IPRP value y (in cm²/MPa·sec) from 1000 to 7700, the MAP value ofthe material (in cm H₂O) is within +1-30% of the value of x in theformula y=mx^(Z); wherein m is from 600 to 1200, and wherein z is from−0.590 to −0.515.
 9. The material of claim 7, characterized in that at agiven IPRP value (in cm²/MPa·sec) from 800 to 5400, the material has aMAP value that is equal to or higher than the corresponding MAP valuepossessed by non-grafted, crosslinked cellulose fiber.
 10. The materialof claim 7, characterized in that at a given MAP value (in cm H₂O) from7.0 to 20, the material has an IPRP value that is equal to or higherthan the corresponding IPRP value possessed by non-grafted, crosslinkedcellulose fiber.
 11. The material claim 1, wherein the material has anIPRP value of 5400 cm²/MPa·sec or above.
 12. A fibrous cellulosicmaterial comprising cellulose-graft-poly(acrylic) acid having a graftyield of 10-20 weight % and intrafiber wherein two or more graftedpolymer chains of poly(acrylic) acid are intra-fiber crosslinked. 13.The material of claim 12, wherein the material has a wet bulk of about15.0-17.0 cm³/g.
 14. The material of claim 13, wherein said two or morepolymer chains of poly(acrylic) acid are intra-fiber crosslinked by oneor more of pentaerythritol and a hyperbranched polymer.
 15. A method ofproducing a grafted, crosslinked cellulosic material, the methodcomprising: grafting polymer chains of at least one monoethylenicallyunsaturated acid group-containing monomer from a cellulosic substrate toproduce a grafted cellulosic material; and subsequently crosslinking thegrafted cellulosic material by treating the material with a crosslinkingagent adapted to effect crosslinking of one or more of the cellulosicsubstrate or the polymer chains.
 16. The method of claim 15, wherein thegrafting is performed in situ.
 17. The method of claim 16, wherein thegrafting includes reacting the at least one monomer with the cellulosicsubstrate in the presence of a grafting initiator.
 18. The method ofclaim 17, wherein the grafting includes varying one or more of theweight percent of the initiator, and the ratio of the cellulosicsubstrate to the monomer, to achieve a desired graft yield level. 19.The method of claim 15, wherein the grafting includes using acrylic acidas the at least one monomer.
 20. The method of claim 15, wherein thecrosslinking includes establishing intra-fiber crosslinks via anesterification reaction, and wherein the crosslinking agent includes oneor more of pentaerythritol, a homopolymer formed of the at least onemonoethylenically unsaturated acid group-containing monomer, and ahyperbranched polymer.